In recent years, the use of fiberoptic networks for the distribution of long distance telecommunication services and local cable access television services has become widespread. In the future, all indications are that the use of fiberoptic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-bandwidth services matures. For instance, such services may include enhanced pay-per-view, video-on-demand, interactive television, interactive games, image networking, video conversing, video telephony, CATV, and ISDN switching services.
As the demand for fiberoptic circuit networks increase, development of new supporting technology and the refinement of existing technology is required for the evolution of the above identified services into reality for end-user subscribers. Several examples of devices representative of the technology developed for implementing fiberoptic networks are dense waveguide division multiplexers (DWDMs), fiber amplifiers such as erbium doped fiber amplifiers (EDFAs), and add/drop networks. As well known in the industry, each of the above devices, in addition to other components of a fiberoptic network, contribute to or are affected by power level variances in the different channels of a fiberoptic link.
For example, in a fiberoptic circuit with cascaded DWDMs and EDFAs, nonequal power levels in the different channels results in poor signal to noise ratio in the low power channels. This is of particular concern because the normal operation of a waveguide grating type DWDM, e.g., the one disclosed in U.S. Pat. No. 5,412,744, induces loss in the outermost channels, a phenomenon commonly referred to as roll-off. In an add-drop network, different channels are combined from different sources, typically having different fiber lengths, and therefore, the power levels on the different combined channels can vary drastically so as to be out of range of the system's tolerances. In an EDFA, the high powered channels saturate the amplifier and drain most of its power so that the channels having lower power are not adequately amplified. Consequently, fiberoptic circuits are presently being designed with more stringent requirements, particularly for devices such as transmitters which are designed to operate at specific power levels and receivers which are designed to have a specific range of sensitivity.
Accordingly, fiberoptic circuits include external attenuators for equalizing the power levels in the channels of a fiberoptic device so as to improve system performance and reliability. However, current configurations of attenuators utilized for power equalization leave much to be desired. First, the attenuators are typically stand alone devices that are incorporated in an optical circuit in a manner that requires one fiber to be coupled at the input of the attenuator and another fiber to be coupled at the output of the attenuator. This is undesirable because it complicates the circuit by the addition of another element which naturally increases the cost and subsequent maintenance of the circuit. Further, with stand alone attenuation of each optical channel, the system is also undesirably bulky.
Second, many of the attenuators in use today for power equalization are not adjustable, or if they are, they are mechanically adjusted. While adjustable (i.e., tunable), attenuators are desirable because they provide for greater control over system performance, the mechanical attenuators take approximately two to three seconds to adjust. This is undesirable because present optical transmission systems respond at near real-time speeds and any disruption of the communication system is very expensive, even if only for a second or two. The present inventor is currently not aware of any inexpensive fiber attenuators that provide real-time adjustment.
Thus, a need exists in the industry that has previously gone unsatisfied for a tunable attenuator that is suitable for integration on the same substrate with an optical device, such as a DWDM, and that can respond in real-time.